The drying of the timber is an obligatory stage for transforming the raw material obtained from sawmills into marketable products. A European directive (which comes into force on 25 Jun. 2005) requires the drying of the timber before its transport, with the objective of avoiding the unnecessary, expensive and polluting transport of the mass of water contained in the raw material.
The drying of the timber is defined by operating regulations which correspond to satisfactory behaviour requirements of the product at the end of the treatment: too rapid drying or at too high a temperature causes lesions (splits, deformations etc.) in the timber, making it unsuitable for the intended use.
The volume of the dryers dedicated to this use ranges from 20 to several hundred cubic meters. The drying of the wood is carried out by the combined action of intense ventilation and heat (circulation of hot air). Each dryer is therefore equipped with its heat system and one or more fans.
Several methods make it possible to dry the wood:                natural drying (in the open air) the timber is stored in a zone (covered or not covered) for several months, or even years. When the desired moisture content is reached the wood is utilized.        Artificial drying which comprises two methodologies:        drying at atmospheric pressure or slightly above,        drying under vacuum.        
In drying at atmospheric pressure or slightly above, the overpressure is obtained by the circulation of a large volume of air heated by a heat generating system (electric resistance heaters, heat pumps, boilers burning wood or other solid fuels, boilers and burners fired by gas or other liquid fossil fuels etc.) The circulation of the volume of hot air through the load of wood to be treated allows the evaporation of the water contained in the wood. The air saturated with this steam is discharged into the atmosphere, in continuously renewable cycles.
The new replacement air comes from the ambient atmosphere and contains a greater or lesser portion of moisture which impairs the yield of the method, the air charged with moisture being more rapidly saturated with steam. The volume of treatment air therefore has to be increased, drying the air improves this yield, but this method is expensive.
The quantity of heat, which is used for the drying, is proportional to the quantity of water to be evaporated (latent heat of evaporation) irrespective of the drying system. When the water, which is extracted from the wood, is condensed after treatment (certain systems do this), the latent heat is rarely exploited, as it is difficult to recover by most of the systems except, perhaps, by heat pumps.
In drying under vacuum, the systems have a reduced treatment capacity, as the pump-down of large volumes is difficult and very expensive. By reducing the environmental pressure of the load of wood to be dried, the evaporation temperature is reduced. Continuous aspiration avoids the internal back pressures in the wood, linked to the evaporation of the water, which allows a better transfer of the water retained internally towards the outside of the wood in order to be evaporated there, and thus more rapid drying. The low pressures created in these systems (pump-down) increase the volume of the steam extracted, in proportion to the degree of negative pressure which must be compensated for by the extraction capacity. The latent heat, provided for the evaporation of the water, is difficult to recycle.
These systems are very significant consumers of heat and electrical energy, which makes their viability problematical. In order to be profitable, these systems must be integrated into programmes with high added value, or receive financial assistance.
The cumulative effect of these factors is that the drying of timber is difficult for professional sectors operating under already highly-adapted economic conditions.